[Bug middle-end/30521] "if (i == n) ++i;" or "i += i == n;"?

[Bug middle-end/30521] "if (i == n) ++i;" or "i += i == n;"?

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30521

--- Comment #8 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-03-04 01:44:32 UTC ---
We could do this at the tree level.  First do add a late PHIOPT which does the
conversion to a COND_EXPR and then simplify a?b+1:b into a+b.  Likewise for
a?b+2:b+1 to a+b+1.

[Bug middle-end/30521] "if (i == n) ++i;" or "i += i == n;"?

[bonzini at gnu dot org]

------- Comment #7 from bonzini at gnu dot org  2008-11-22 10:15 -------
Subject: Re:  "if (i == n) ++i;" or "i += i == n;"?


> The form of the code before CSE is caught in noce_try_addcc. The second form
> obviously not (because we don't know that the value of i.20 is "i + 1").
> 
> So this comes down to a pass ordering problem.

I'll just point out that the df merge allowed much better freedom in
pass ordering.  Ifconv1 could be anticipated before CSE and fwprop.

Paolo


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30521

[Bug middle-end/30521] "if (i == n) ++i;" or "i += i == n;"?

[steven at gcc dot gnu dot org]

------- Comment #6 from steven at gcc dot gnu dot org  2008-11-22 10:04 -------
Ah, now I see what Pinski meant at comment #2.

Before CSE, we still have the original code for f1:
unsigned int f(unsigned int i, unsigned int n)
{
  i.20 = i + 1;
  if (i.20 == n) i.20 = i.20 + 1;
  return i.20;
}

After CSE (but before the first if-conversion pass) it's been transformed to:
unsigned int f(unsigned int i, unsigned int n)
{
  i.20 = i + 1;
  if (i.20 == n) i.20 = i + 2;
  return i.20;
}

The form of the code before CSE is caught in noce_try_addcc. The second form
obviously not (because we don't know that the value of i.20 is "i + 1").

So this comes down to a pass ordering problem. Or, one  could argue that CSE
should not perform this transformation if (say) "i.20 = i + 1" is not made dead
code by the transformation to "i.20 = i + 2".


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30521

[Bug middle-end/30521] "if (i == n) ++i;" or "i += i == n;"?

[steven at gcc dot gnu dot org]

------- Comment #5 from steven at gcc dot gnu dot org  2008-11-22 09:51 -------
I created a t.c with both functions in it:

unsigned int f1(unsigned int i, unsigned int n) {++i; if (i == n) ++i; return
i;}
unsigned int f2(unsigned int i, unsigned int n) {++i; i += i == n; return i;}

With today's trunk on x86_64, I get:

        .file   "t.c"
        .text
        .align 16
.globl f1
        .type   f1, @function
f1:
.LFB0:
        leal    1(%rdi), %eax
        addl    $2, %edi
        cmpl    %esi, %eax
        cmove   %edi, %eax
        ret
.LFE0:
        .size   f1, .-f1
        .align 16
.globl f2
        .type   f2, @function
f2:
.LFB1:
        addl    $1, %edi
        xorl    %eax, %eax
        cmpl    %esi, %edi
        sete    %al
        addl    %edi, %eax
        ret
.LFE1:
        .size   f2, .-f2
        .section        .eh_frame,"aw",@progbits
.Lframe1:
        .long   .LECIE1-.LSCIE1
.LSCIE1:
        .long   0x0
        .byte   0x1
        .string "zR"
        .byte   0x1
        .byte   0x78
        .byte   0x10
        .byte   0x1
        .byte   0x3
        .byte   0xc
        .byte   0x7
        .byte   0x8
        .byte   0x11
        .byte   0x10
        .byte   0x1
        .align 8
.LECIE1:
.LSFDE1:
        .long   .LEFDE1-.LASFDE1
.LASFDE1:
        .long   .LASFDE1-.Lframe1
        .long   .LFB0
        .long   .LFE0-.LFB0
        .byte   0x0
        .align 8
.LEFDE1:
.LSFDE3:
        .long   .LEFDE3-.LASFDE3
.LASFDE3:
        .long   .LASFDE3-.Lframe1
        .long   .LFB1
        .long   .LFE1-.LFB1
        .byte   0x0
        .align 8
.LEFDE3:
        .ident  "GCC: (GNU) 4.4.0 20081122 (experimental) [trunk revision
142117]"
        .section        .note.GNU-stack,"",@progbits


So still not the same functions.

For x86 (32bit) with -march=core2, I get similar code (f1 with a cmove).
Without a -march option, I get code with a jump.


-- 

steven at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2007-01-22 00:29:40         |2008-11-22 09:51:57
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30521

[Bug middle-end/30521] "if (i == n) ++i;" or "i += i == n;"?

[bonzini at gnu dot org]

------- Comment #3 from bonzini at gnu dot org  2008-08-27 04:40 -------
*** Bug 37240 has been marked as a duplicate of this bug. ***


-- 

bonzini at gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bonzini at gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30521

[Bug middle-end/30521] "if (i == n) ++i;" or "i += i == n;"?

[bonzini at gnu dot org]

------- Comment #4 from bonzini at gnu dot org  2008-08-27 04:40 -------
see PR37240 for another testcase.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30521

[Bug middle-end/30521] "if (i == n) ++i;" or "i += i == n;"?

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2007-01-22 00:29 -------
CSE is pushing i+=1; down into the conditional which causes ifcvt not to catch
the  if(i) i+=1;.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
 GCC target triplet|                            |i?86-linux
           Keywords|                            |missed-optimization
   Last reconfirmed|0000-00-00 00:00:00         |2007-01-22 00:29:40
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30521

[Bug middle-end/30521] "if (i == n) ++i;" or "i += i == n;"?

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2007-01-21 08:54 -------
I think this has been fixed already, for PPC with 4.0.2, we get:
_f:
        mr r2,r3
        addi r3,r3,1
        cmpw cr7,r3,r4
        bnelr cr7
        addi r3,r2,2
        blr
        .align 2
        .p2align 4,,15
        .globl _f1
_f1:
        addi r3,r3,1
        xor r0,r3,r4
        subfic r0,r0,0
        addze r0,r3
        mr r3,r0
        blr


While on the trunk we get:
_f:
        addi r3,r3,1
        xor r0,r3,r4
        subfic r0,r0,0
        addze r0,r3
        mr r3,r0
        blr
        .align 2
        .globl _f1
_f1:
        addi r3,r3,1
        xor r0,r3,r4
        subfic r0,r0,0
        addze r0,r3
        mr r3,r0
        blr


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30521